The epithelium of the intestine forms the largest interface between the sterile core of the body and the outside, and it also forms the main entry port for pathogens. Preserving the integrity of the mucosal barrier is one of the major challenges of the mucosal immune system, which consists largely of T cells that reside within the epithelium. Most of these T cells are CD8+cytotoxic lymphocytes (CTL), whereas resident CD4 T cells are less frequent. However, under inflammatory conditions, peripheral CD4 T helper (Th) cells infiltrate and cause epithelial damage leading to severe chronic inflammation. We recently discovered a novel type of CD4 T cells that interestingly had much more in common with the cytotoxic CD8 T cells as compared to the typical CD4 T helper cells. We further determined that the differentiation of this novel type of CD4 T cells is controlled by the activation-induced loss f the key transcription factor, ThPOK, shown to be indispensable to maintain the Th phenotype in mature CD4 T lymphocytes. CD4 T cels that differentiate along this novel pathway lose the Th features but gain CD8 CTL-like functions, which enable them with protective and regulatory trades. The notion that mature CD4 T lymphocytes can be functionally redirected, represents significant opportunities for new medical interventions, not only for the treatment of intestinal inflammatory diseases, but equally important for the design of new vaccination strategies that aim at generating pre- existing pathogen- or tumor-specific cytotoxic MHC class II restricted immunity. Because these effector cells maintain their MHC class II restriction, it renders them potentially capable of detaining viral infections tropic for class II+ target cells, including infeted MHC class II+ intestinal epithelial cells (IEC), but also various transformed cells. Furthermore, such MHC class II restricted effector cells might also be critical to fight MHC class I negative tumors or protect against viruses, including HIV-1, that have developed specific mechanisms to escape surveillance by MHC class I restricted CD8 CTL. Because of this broad potential of these newly defined CD4 effector cells for their use in medical interventions to treat or prevent multiple health threats, it is of utmost importance to fully understand the mechanisms and factors involved, that lead to the differentiation of these cels in vivo. The comprehensive study that we propose here is designed to form the basis of a whole new field of research that aims at understanding the potential of these T cells and the application of their beneficial functions to treat/prevent disease, including inflammatory- or infectious diseases and cancers. PUBLIC HEALTH RELEVANCE: We have uncovered a novel type of CD4 T lymphocytes that are closely related to cytotoxic CD8 T cells. The process that uniquely differentiates these CD4 T cells is mediated by the activation-induced loss of a key transcription factor that normally keeps these CD4 T cells in the T helper lineage. This alternative differentiation also prevents the CD4 T cells from becoming pathogenic inflammatory cells. These new insights have very important implications for our understanding of T cell biology and for medical interventions. The proposed study here, aims to identify mechanisms and factors responsible for the generation of this novel type of immune effector cells. Information gained from this research will undoubtedly lead to novel and/or improved strategies to combat devastating diseases such as infections, inflammatory diseases and cancers.